diff --git a/icu4c/source/data/mappings/gb18030-2022.ucm b/icu4c/source/data/mappings/gb18030-2022.ucm
index c85538940a55..7f02d06856bf 100644
--- a/icu4c/source/data/mappings/gb18030-2022.ucm
+++ b/icu4c/source/data/mappings/gb18030-2022.ucm
@@ -5,6 +5,12 @@
 
 # ICU codepage data for GB 18030-2022
 
+# This data file was originally generated from the mapping tables
+# published with the original (year 2000) GB18030 standard.
+# It has been updated for the 2005 version of GB18030 (ICU-8274 & ICU-8427)
+# and for the 2022 version (ICU-22357).
+# ICU-22420 then made minor mapping changes for GBK and web data/WHATWG compatibility.
+
 <code_set_name>               "gb18030-2022"
 <char_name_mask>              "AXXXX"
 <mb_cur_max>                  4
@@ -23,7 +29,8 @@
 # The second <icu:state> line is commented out (and does not count)
 # because the state table is hand-optimized and does not use what would be
 # the natural path for the encoding scheme.
-<icu:state> 0-7f, 81:6, 82:7, 83:8, 84:9, 85-fe:3
+# ICU-22420 makes 0x80 valid for the GBK encoding of the Euro sign.
+<icu:state> 0-80, 81:6, 82:7, 83:8, 84:9, 85-fe:3
 # <icu:state> 30-39:2, 40-7e, 80-fe
 <icu:state> 81-fe:2
 <icu:state> 30-39
@@ -56,6 +63,18 @@
 
 CHARMAP
 
+# ICU-22420 reverse fallbacks for compatibility with GBK and other web data as in WHATWG.
+# U+20AC = EURO SIGN (normally \xA2\xE3)
+# U+3000 = IDEOGRAPHIC SPACE (normally \xA1\xA1)
+#
+# PUA U+E5E5 used to round-trip to \xA3\xA0, as specified in GB18030.
+# Now that \xA3\xA0 maps to U+3000 (“reverse fallback” mapping),
+# we use a “good one-way” mapping from U+E5E5 to \xA3\xA0
+# for maximum compatibility with previous behavior.
+<U20AC> \x80 |3
+<U3000> \xA3\xA0 |3
+<UE5E5> \xA3\xA0 |4
+
 <U0000> \x00 |0
 <U0001> \x01 |0
 <U0002> \x02 |0
@@ -29602,7 +29621,7 @@ CHARMAP
 <UE5E2> \xA3\x9D |0
 <UE5E3> \xA3\x9E |0
 <UE5E4> \xA3\x9F |0
-<UE5E5> \xA3\xA0 |0
+# <UE5E5> \xA3\xA0 |0
 <UE5E6> \xA4\x40 |0
 <UE5E7> \xA4\x41 |0
 <UE5E8> \xA4\x42 |0
diff --git a/icu4c/source/test/testdata/conversion.txt b/icu4c/source/test/testdata/conversion.txt
index 290925ef31fa..8b430a53f89a 100644
--- a/icu4c/source/test/testdata/conversion.txt
+++ b/icu4c/source/test/testdata/conversion.txt
@@ -115,6 +115,14 @@ conversion:table(nofallback) {
           :intvector{ 0,1,5,5,5,5,6,7,9,9,9,9,10,11,12,13,13,13,13,14,15,17,18,20 },
           :int{1}, :int{0}, "", "&C", :bin{""}
         }
+        // GB18030: ICU-22420 adds two reverse fallbacks
+        {
+          "gb18030",
+          :bin{ 80a1a1a2e3a3a0 },
+          "\u20AC\u3000\u20AC\u3000",
+          :intvector{ 0,1,3,5 },
+          :int{1}, :int{0}, "", "&C", :bin{""}
+        }
         {
           "UTF-8",
           :bin{ 61f1808182f180813cf18081fff180ff3cf1ff3c3e7a },
diff --git a/icu4j/main/charset/src/main/resources/com/ibm/icu/impl/data/icudt74b/gb18030-2022.cnv b/icu4j/main/charset/src/main/resources/com/ibm/icu/impl/data/icudt74b/gb18030-2022.cnv
index 4731af8d1f1b..fdc914760004 100644
Binary files a/icu4j/main/charset/src/main/resources/com/ibm/icu/impl/data/icudt74b/gb18030-2022.cnv and b/icu4j/main/charset/src/main/resources/com/ibm/icu/impl/data/icudt74b/gb18030-2022.cnv differ
diff --git a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/data/testdata/conversion.res b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/data/testdata/conversion.res
index a61b4b8d676c..8d66e5616880 100644
Binary files a/icu4j/main/core/src/test/resources/com/ibm/icu/dev/data/testdata/conversion.res and b/icu4j/main/core/src/test/resources/com/ibm/icu/dev/data/testdata/conversion.res differ